Dissolved air flotation system

ABSTRACT

A dissolved air flotation system and method for purifying fresh water. The system is self-contained within a standard shipping container, with all components disposed in-line within the shipping container. Float is removed from the flotation tank with a skimmer that drives float downstream onto a conveyor belt, and the conveyor belt conveys float away from the flotation tank, in the downstream direction, and deposits the float in a collection tank. Clean water may be drawn from the flotation tank continuously, while float may be removed periodically.

FIELD OF THE INVENTIONS

The inventions described below relate the field of dissolved airflotation.

BACKGROUND OF THE INVENTIONS

Dissolved air flotation is one of several processes used to clean andpurify water. The process may be used for removing suspended solids,including organisms, from fresh water for municipal water supplies, orfor removing suspended solids from industrial wastewater. In thedissolved air flotation process, influent water is mixed with acoagulant to form flocculent particles, and then white water(air-saturated high-pressure water) is injected into a tank of theinfluent water/flocculent mixture to form air bubbles that attach to theflocculent particles and lift them to the surface. The particles liftedto the surface accumulate in a layer of scum or sludge, referred to asthe float. This layer is skimmed off the surface of the tank, whileclean water is drawn out of the bottom of the tank.

For municipal water supplies, the suspended solids of concern includemicrobial pathogens such as cryptosporidium and giardia. Cryptosporidumand giardia are water-borne protozoan parasites that cause intestinalinfections. In typical municipal purification systems, chlorine is usedto kill these organisms, even after filtration and other purificationmethods may have removed a large portion of the influent population.This results in the formation of trihalomethanes (chloroform,dichlorobromomethane, dibromochloromethane, and bromoform) when thechlorine reacts with compounds already in the fresh water. Thesecarcinogenic disinfection by-products are clearly undesirable. Reductionof microbial pathogens prior to chemical treatment in the water isdesirable in order to minimize the creation of disinfection by-products.

Though dissolved air flotation is promising as a technique for removingmicrobes and other suspended solids, its use has been hampered by lowvolume, lack of need given lax water quality standards, and high cost ofinstallation. These limitations are overcome by the new system describebelow.

SUMMARY

The systems and methods described below provide for efficient, modular,transportable dissolved air flotation systems. The system includes awaste transport system that permits continuous operation, entailsminimal loss or diversion of influent water for float removal, andprovides for easy disposal of waste materials in the float.

The dissolved air flotation system described below removes 99.99% ofparticles in the size range of 1 to 10 microns. This represents a 4 to 5log removal rate. Particles in the 1 to 10 micron size range includeprevalent fresh water contaminants such as cryptosporidium (3-5 microns)and giardia (5-7 microns), escherichia coli, and microscopic organicalgae and protozoa. The clean effluent may then be treated inconventional disinfectant and filtering systems with greatly reducedload on those systems. Thus, the material requirements for these systemsis lower, and the need for injection of chlorine is minimized, alongwith the deleterious production of toxic disinfectant by-products(voluble organic chemicals) which are the result of chlorine injectioninto water with high bacterial content. Thus, use of the system willminimize the risk from both microbial pathogens and disinfectantby-products simultaneously.

The system includes a flocculation tank, flotation tank, wastecollecting system, filtering system, disinfecting system and controlsystem arranged in a linear arrangement, with all the components sizedand dimensioned to fit within a standard shipping container. The wastecollecting system includes a skimmer that moves across the surface ofthe water in the flotation tank and pushes the float onto a beach. Thebeach is provided in the form of a conveyor belt, which conveys skimmedfloat away from the tank and deposits it in a collecting tank. Theskimmer is driven over the surface of the water, and the conveyor beltis driven at a speed that matches or slightly exceeds the skimmer speed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the dissolved air flotation system adapted forinstallation in a standard shipping container.

FIG. 2 shows a side view of the system.

FIG. 3 is a detailed view of the beach and waste transport system.

DETAILED DESCRIPTION OF THE INVENTIONS

FIGS. 1 and 2 illustrate the dissolved air flotation system 1. The majorcomponents of the dissolved air flotation system include a flotationtank 2 and a dissolved air supply 3, a flocculation tank 4 andinfluent/coagulant mixing system 5, a skimmer assembly 6, a wastecollecting system that includes a beach 7 and waste transport system 8,post-treatment filtration tanks 9, and a control room 10.

The flotation tank 1 is used to mix influent water with a stream ofwater containing a large amount of dissolved air. The bubbles producedfrom the dissolved air attach to flocculent (particles suspended in thewater which have aggregated into clumps or masses in the flocculationtank) grown in the influent water and floats to the top of the tankwhere it collects in a thin layer referred to as float, sludge, or scum.The flotation tank is supplied with unpurified influent from theflocculation tank, flowing over weir 11 (as influent is pumped into theflocculation tank, the flocculation tank overflows into the flotationtank), and is supplied with high dissolved air content “white water”through supply line 12 which feeds a manifold 13 of injection portslocated at the bottom of the flotation tank. Clean water is removed fromthe flotation tank through clean water outputs 14 (in fluidcommunication with output manifolds disposed transversely in the lowerdownstream portion of the flotation tank) and clean water output line15. The flotation tank measures about 5 feet wide, 12 long, and 7 feetdeep, and holds about 3000 gallons when full.

The flocculation tank 2 is used to combine coagulant molecules orparticles with impurities in the influent water. The flocculation tankis supplied with influent fresh water which has been mixed withcoagulant and, optionally, conditioning chemicals used to adjust waterchemistry to enhance flocculation. Influent fresh water is injected intothe flocculation tank through influent supply line 16 at the lowercorner. The influent water thereafter flows through a tortuous pathwaythrough the tank (the tortuous pathway is created by baffles disposedwithin the tank), until it overflows over the dam or weir 11 into theflotation tank, so as to provide adequate dwell time for theflocculation of the coagulant and impurities in the influent water. Theweir serves to limit the flow of water from the flocculation tank to theflotation tank, until the water level in the flocculation tank exceedthe height of the weir. The weir may be formed from the downstream wallof the flocculation tank, and, correspondingly, the upstream wall of theflotation tank. Prior to injection, influent/coagulant mixing system 5draws and mixes a small amount of coagulant into the influent stream(coagulant concentration of 1-100 ppm is sufficient). The coagulant isstored in a small tank 17, and is mixed. The flocculation tank measuresabout 5 feet wide, 9 feet long, and 7 feet deep, and holds about 1800gallons when full.

The float is removed by the skimmer assembly 6. The skimmer assemblycomprises a flight 19 which is driven across the surface of the water inthe flotation tank by a flight drive mechanism 20. The flight extendstransversely across the flotation tank, and preferably extends fromsidewall to sidewall with little or no clearance. Conformable wipers orseals may be applied to the transverse ends of the skimmer to contactand wipe the sidewalls during skimmer translation to prevent retrogradeflow of float. The flight may also be referred to as a skimmer. Thedrive mechanism may be a chain drive, worm gear, or any other suitabledrive mechanism. Prior to movement of the flight, flow into and out ofthe system is adjusted to allow the flotation tank water level to riseabove the level of the downstream wall of the flotation tank. The flightis moved slowly, at about 1.5 feet per minute (about 0.5 meters perminute) to push the float toward the downstream end of the flotationtank without causing significant turbulence in the flotation tank. Thefloat is pushed over the end wall, and out of the tank. After the flighthas skimmed the entire length of the flotation tank, it is drivenfurther past the end of the tank, and is raised or tilted upward andwashed. Also, the tank level is drawn down, below the level of the endwall. After washing, the flight is drawn back to the upstream end of thetank, where it is stored in anticipation of the next pass. During theskimmer pass, the water inflow, white water injection, and outputcontinue.

The skimmer pushes the float over the end wall of the flotation tank.The float is pushed onto the beach 7 and waste transport system 8. Thebeach is provided in the form of a rolling conveyor belt. The beltcontinuously travels away from the flotation tank, and transportsdeposited float away from the tank. The float falls off the belt as thebelt turns under rollers during its travel, and falls into a wastecollector pan 21. The beach is rolled at a speed matching, or slightlyexceeding, the horizontal speed of the flight. For example, belt speedmay be 1.5 feet per minute to 2 feet per minute, and should be no lessthan 100% of belt speed, and is preferably about 105% to 120% of skimmerspeed. By slightly exceeding the flight speed, the float is removed at arate that prevents build-up on the belt (and potential retrograde flowback over the end wall into the flotation tank), while also avoidingturbulence in the flow. Float in the waste collector pan flows throughwaste line 22 to storage tanks or other systems for processing. Thewastewater can be very fluid, though it contains up to 10% solids.Depending on the contaminants in the effluent water and the coagulantused, the solid waste can be processed for other uses, such asfertilizer or fuel. (It is expected that sediment will be minimal whenthe system is used to purify fresh water supply to municipal watersystems, so that the bottom of the tank need not be wiped regularly, butmay be vacuumed yearly or quarterly.)

FIG. 2 shows a side view of the system. In this view, the internal partsof the flocculation tank and flotation tank are more clearlyillustrated. In the flocculation tank, the influent inlet 23 ispositioned at the bottom of the tank, and the upstream end of the tank.Several baffles 24 are positioned to create the tortuous pathway desiredto increase dwell time and encourage flocculation (the agglomeration ofcoagulant molecules and suspended solids). In the inlet piping, acoagulant supply tank and appropriate piping provided coagulant to theinfluent stream, and this is thoroughly mixed in the influent/coagulantmixing system 5. After mixing and flocculation in the flocculation tank,the flocculent-containing water flows over weir 11 into the flotationtank. The flotation tank contains an inflow baffle 25, which directsinflowing water downwardly, toward the aerated water (white water) inletmanifold 13. The mixed white water and flocculated influent flowupwardly through the bubble contact zone 27 established by the baffles25 and 28, and then into the flotation zone 29. In the bubble zone, thedissolved air in the white water comes out of solution and formsmicro-bubbles, and these micro-bubbles attach to the flocculentparticles to form buoyant agglomerations of bubbles and flocculentparticles. In the flotation zone, the bubble/flocculent agglomerationsfloat to the surface of the water in the tank, and remain floating onthe surface until skimmed from the tank by the flight. Clean water isremoved from the bottom of the flotation tank. A small portion of theclean water may be diverted to the white water system, to be aerated andinjected back into the flotation tank. The remainder of the clean wateris pumped by pump 30 to filter tanks 9 and thereafter to disinfectionsystem 31, after which it is considered potable water, and thereafterthrough potable water output line 32 into the municipal water supply orpotable water supply. The filters may comprise filter tanks may comprisemicro-filtration systems ultra-filtration systems, and the disinfectionsystem may comprises an ultraviolet disinfection system or a chlorineinjection system. The load on the disinfection system and filter tanksis greatly reduced by the removal of almost all the suspended solids inthe influent water. The operation of the system, including monitoring ofwater chemistry, pump performance and flow measurements, periodicover-filling of the flotation tanks, and periodic translation of theflight, may be controlled from control mechanisms housed in the controlroom 10. The system is preferably automated and controlled by computer,and/or requiring only routine monitoring by system operators andtechnicians.

All of the components are adapted for installation in a standardshipping container 40. The standard shipping container is preferablyadapted for transport on rail cars, trucking platforms and containerships. The container is, thus, a standard size of 40 feet long, 8 feetwide, and 8 feet tall to match standard-sized shipping containers in usein the United States (the dimensions may be varied slightly while stillmaintaining the preferred compatibility with various trucking, rail, andshipping standards, and still be considered standard size). The shippingcontainer is modified with the addition of closures necessary for accessto the various subsystems of the purification system. The size may beadjusted to meet differing regional standards. All the components of thesystem fit within the container, so that the installation site need onlysupply electrical power and influent water, and replenishment of theconsumables used in the process. Providing the system in the containerallows the systems to be built in a central location, and thereafter betransported to any site for use. Multiple systems may be ganged togetherto meet any need, and thus may be purchased as needed by municipal watersuppliers to meet the needs of growing communities. The systems may beshipped anywhere, and thus may be provided to remote locations quickly.

FIG. 3 is a detailed view of the beach and waste transport system. Thebeach comprises a continuous belt 40, disposed over drive spool 41 andspindles 42 and 43. The float receiving surface 44 of the belt ispreferably inclined upwardly, from upstream to downstream, at a slightangle of about 7 degrees. The drive spool and spindles are held in fixedrelationship of each other by frame 45 on either side of the belt. Theframes are boomerang shaped or L-shaped, so that they hold the threeshafts in triangular relationship to each other, and is mounted, asshown in FIG. 1, to hold the beach surface (the upstream upper surfaceof the belt) is proximate the downstream wall of the flotation tank sothat float pushed from the tank is deposited on this surface, and alsoto hold the beach surface at a slight upward incline (from upstream todownstream). The drive spool may be driven by a chain, through drivegear 46 shown in FIG. 1. The relative sizes of the drive gear and drivespool, and the flight drive gear, are chosen such that the belt travels,as mentioned above, slightly faster than the flight. The drivemechanisms may of course be provided in the form of a worm gear or gearsoperably connected to the beach drive spool and the flight, orindividual motors directly driving the beach drive spool and the flight.The conveyor belt may be provided in other forms, for example movingtraversely to a collection tank mounted, but the downstream direction ispreferred.

At the downstream end of the belt, a manifold of spray nozzles 49 isprovided to wash the belt continuously as it rolls to remove any floatthat has not fallen from the belt. A second manifold of spray nozzles 48is provided to wash the flight after each pass. The belt is fabricatedfrom fabric such as woven polypropylene or PVC fabric (with laminatedborders), or other suitable material, and is preferably water permeable,so that wastewater can filter through the belt during transport (so thatthe belt acts as a filter belt). The belt may be provided in adisposable roll of material that is spooled from a large roll, over thespindles, and thereafter onto a collector spindle, but the continuousbelt is expected to last for months in full capacity operation.

Operation of the system is preferably controlled by a computer controlsystem in the control room. Inflow rates and tank levels are controlledby the computer system, through the control of various pumps and valvesthroughout the system. Control may be accomplished by providing pump 30in the form of a variable speed pump, with appropriate control means inthe control system, by providing appropriate throttle valves in theinput and output lines, or other suitable means. The growth of the floatmay be sensed by sensors in or near the flotation tank, and thesesensors are operably connected to the computer control system to providesignals corresponding to float thickness to the control system. Thecomputer control system is programmed to receive and interpret inputfrom the sensors, and is also programmed to operate various subsystemsto overfill the flotation tank, drive the skimmer across the tank, andoperate the conveyor belt when information from the sensors indicatesthat the float thickness is of a predetermined thickness. In a simplerembodiment, the skimmer movement may performed at predetermined timeintervals or upon predetermined chemical (coagulant or other chemicals)consumption. The length of time between skims, or the amount of chemicalconsumption between skims, will vary depending on the type andconcentration of suspended solids in the influent water. Time intervalsare measured by the control system clock, and chemical consumption canbe monitored by flow sensors or tank level sensors provided in the tanksused to hold the chemicals. These sensors are operably connected to thecomputer control system to provide signals corresponding signals, andmay be used by the control system to operate various subsystems tooverfill the flotation tank, drive the skimmer across the tank, andoperate the conveyor belt when information from the clock or sensorsindicates that the predetermined conditions for skimming the flotationtank have been met. The control system also controls the lifting of theskimmer at the end of a skim and operation of one spray manifold toclean the skimmer, and controls operation of the other spray manifold toclean the downstream portion of the conveyor belt while it is moving.The control system also receives, monitors, and report various measuredparameters of the system, such as fluid flow and pressure, turbidity andtemperature, tank levels, etc.

When constructed and operated as described above, the system providespotable water at a rate of about 200 gallons per minute, or 288,000gallons per day. This would supply potable water for about 600households in the United States.

While the inventive systems and methods have been described in relationto dissolved air flotation, it may be used for rudimentary air flotationsystems (similar systems using macroscopic bubbles in the flotationtank) as well. Also, although described in relation to purifying freshwater influent for use in municipal water supplies, the beneficialaspects of the system may be employed in wastewater systems for avariety of industries. Thus, while the preferred embodiments of thedevices and methods have been described in reference to the environmentin which they were developed, they are merely illustrative of theprinciples of the inventions. Other embodiments and configurations maybe devised without departing from the spirit of the inventions and thescope of the appended claims.

1. A self contained dissolved air flotation system for purifyinginfluent water, said system comprising: a flocculation tank; a flotationtank having an upstream end and a downstream end, and correspondingupstream end wall and downstream end wall; an influent water supply portin fluid communication with the flocculation tank and a flocculationtank output communicating downstream to the upstream end of theflotation tank; a skimmer disposed above the flotation tank, saidskimmer extending transversely across the flotation tank; means fordriving the skimmer from the upstream end of the flotation tank to thedownstream end of the flotation tank, thereby pushing float out of thedownstream end the flotation tank; a conveyor belt disposed downstreamof the downstream end of the flotation tank, positioned relative to theflotation tank to receive float pushed from the flotation tank, saidconveyor belt being operable to convey float away from the flotationtank, in the downstream direction; a float collection tank disposedrelative to the conveyor belt to receive float from the conveyor belt;and output manifolds disposed in the lower portion of the downstream endof the flotation tank for drawing clean water from the flotation tank;wherein the skimmer is driven over the flotation tank at a first speed,and the conveyor belt is driven at a second speed slightly higher thanthe first speed.
 2. The system of claim 1 wherein the skimmer is drivenover the flotation tank at about 1.5 feet per minute, and the conveyorbelt is driven at about 1.5 to 2 feet per minute.
 3. The system of claim1 further comprising: a sensor for sensing the thickness of a floataccumulated on the surface of water in the flotation tank, said sensorbeing operable to produce a signal corresponding to float thickness; acontrol system operable to receive and interpret the signalcorresponding to float thickness, said control system programmed todetermine when float thickness reaches a predetermined thickness, andthereupon operate the skim to translate along the length of theflotation tank and correspondingly operate the conveyor belt to conveyfloat deposited thereon into the float collection tank; a first spraymanifold for cleaning the skimmier after it has been driven to thedownstream end of the flotation tank, said spray manifold locateddownstream of the flotation tank; a second spray manifold for sprayingthe downstream end of the conveyor belt, said manifold being disposedproximate the downstream end of the conveyor belt; wherein the controlsystem is programed to operate the first spray manifold to clean theskimmer when the skimmer is disposed proximate the downstream end of theflotation tank, and to operate the second spray manifold when theconveyor belt is moving to convey float away from the flotation tank. 4.The system of claim 1 or 2 further comprising: level control means forcontrolling the water level in the flotation tank; wherein the controlsystem is programmed to operate the level control means during operationof the system, to overfill the tank so that water lever rises above thedownstream end of the flotation tank.